Human monocytes and macrophages, collectively termed the mononuclear phagocytic system, are intimately involved in a variety of physiological and pathological events. They participate in the inflammatory process, secrete numerous biologically active products and are major effectors of immunity against neoplastic and infectious diseases. Their capacity to carry out these functions can be enhanced by a variety of stimuli which activate these cells. This study will define the physiologic changes which occur during the differentiation process that results in activation of the human monocyte. These studies will compare quantitative functional and biochemical activities of normal adult and neonatal monocytes and human peritoneal macrophages with those of in vitro activated monocytes. Monocytes will be activated with lymphocyte products generated in both toxoplasma and Con A in vitro systems. This information will provide this background for the subsequent evaluation of patients with diseases which may be associated with depressed or enhanced monocyte function (leukemia, bacteremia, tuberculosis). The parameters to be examined in mononuclear phagocytes include: 5' nucleotidase activity, enzyme secretion, pinocytic and metabolic rates, phagocytosis and antimicrobial activity. An understanding of monocyte function in health and disease should enhance our ability to manipulate and monitor the cellular immune system to the advantage of the patient.